Control system for hydraulic devices

ABSTRACT

A control system for a hydraulic ram carried on a forklift truck comprises a manually operable control valve which controls fluid flow between a fluid reservoir and the ram. Moving the valve to open (raise) position permits fluid to flow from the reservoir to the ram and simultaneously closes an electric switch which effects, through a relay and a contactor, closure of the contacts of a motor controller which connect an electric pump motor to a battery. The pump supplies pressurized fluid from the reservoir to the ram. Closure of the switch also energizes a capacitor which discharges to maintain the relay energized and the motor controller contacts closed until the pump motor gets up to full speed, even though the valve is moved from open (raise) position and the electric switch is opened. By this means the motor controller contacts are prevented from opening on heavy inrush currents which occur while the motor is coming up to full speed and which would damage the contacts. As a further protection, the contacts are shunted by another capacitor which absorbs energy as the contacts open on normal line current conditions.

United States Patent m1 [72] Inventor Luke F. Henry Primary Examiner Edgar W. Geoghegan Homewood, Ill. AttorneysThomas F. Kirby, Charles L. Schwab and Robert [21] Appl. No. 42,199 B. Benson [22] Filed June 1, 1970 [45] Patented Dec. 28, 1971 73 Assignee u c i Ma f t i Company ABSTRACT: A control system for a hydraulic ram carried on Mil k Wi a forklift truck comprises a manually operable control valve which controls fluid flow between a fluid reservoir and the ram. Moving the valve to open (raise) position permits fluid to [54] CONTROL SYSTEM FOR HYDRAULIC DEVICES flow from the reservoir to the ram and simultaneously closes 4 Claims, 1 Drawing Fig. an electric switch whlch effects, through a relay and a contactor, closure of the contacts of a motor controller which con- [52] US. Cl. 60/52 HE, nee an l i pump motor to a battery. The pump supplies 187/9 pressurized fluid from the reservoir to the ram. Closure of the [51] Int. Cl F151) 15/18 it h 1 ene izes a capacitor which discharges to maintain [50] Field of Search 60/DIG. 2, th relay energized and the motor controller contacts closed 52 HE; 187/9 until the pump motor gets up to full speed, even though the valve is moved from open (raise) position and the electric [56] References (med switch is opened. By this means the motor controller contacts UNITED STATES PATENTS are prevented from opening on heavy inrush currents which 2,968,927 1/1961 Quayle 60/51 X occur while the motor is coming up to full speed and which 2,877,626 3/1959 Gratzmuller 60/52 would damage the contacts. As a further protection, the con- 2,956,550 10/1960 Hippie 6OID1G. 2 tacts are shunted by another capacitor which absorbs energy 3,443,380 5/1969 Krazija 60/D1G. 2 as the contacts open on normal line current conditions.

HYDRAULIC RESERVOIR CONTROL SYSTEM FOR HYDRAULIC DEVICES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to control systems for hydraulic devices such as hydraulic rams. More particularly, it relates to such control systems which include cooperable hydraulic and electrical elements and to means for reducing arcing on the electrical contacts which control operation of a pump motor which delivers hydraulic fluid to the hydraulic device.

2. Description of the Prior Art In some equipment, such as forklift trucks, for example, a motor-driven pump connected to a hydraulic fluid reservoir furnishes fluid under pressure to operate a lift ram which raises and lowers a lift carriage. In such equipment, the driver manually operates a lever to open a valve which connects the pump to the lift ram. Opening of the valve simultaneously closes an electric switch which operates a motor controller and causes energization of the pump motor from a battery. The pump then supplies fluid under pressure to the lift ram through the open valve. Heretofore, closing of the switch connected the contactor coil of the motor controller directly to the battery. The normally open contactor contacts when closed and connected the motor to the battery. However, with the valve open and with the pump driving against a load, a time interval had to elapse before the motor came up to full speed and inrush current thereto would diminish. However, if the driver moved the lever to close the valve and open the switch before the motor was at full speed, the contactor contacts then opened on high inrush current and were subjected to severe arcing and consequent damage which required their frequent replacement. It is desirable, therefore, to provide improved control systems for hydraulic devices which overcome these problems and have other advantages.

SUMMARY OF THE INVENTION A control system for a hydraulic device in accordance with the present invention is employed to advantage with the hydraulic lift ram on a mobile lift truck. The system contemplates a hydraulic fluid source, such as a reservoir, a pump downstream of the reservoir for supplying pressurized fluid to the ram, a manually operable valve downstream of the pump and movable to a lift position wherein it permits fluid flow to the ram, an electric power source, such as a storage battery, an electric motor for driving the pump, motor control means comprising a relay and a contactor having separable contacts for connecting and disconnecting the motor to and from the battery, a switch connected to the manually operable valve and movable in response to movement of the valve to a position wherein it effects closing of the separable contacts and connection of the motor to the battery whereby the pump supplies fluid under pressure to said device, and time delay means, including a capacitor and a resistor in circuit with the relay, operable in response to operation of the switch to maintain the relay contacts and the contactor contacts closed for a predetermined length of time after the valve and switch are moved from lift" position.

OBJECTS OF THE INVENTION It is an object of the present invention to provide improved control systems for hydraulic devices, such as hydraulic rams employed on mobile or self-propelled lift trucks.

Another object is to provide control systems of the aforesaid character which employ a pump driven by an electric motor and wherein the contacts which control motor operation are prevented from being subjected to interrupting inrush currents and arcing upon separation.

Another object is to provide improved means for maintaining such motor control contacts closed until the pump motor obtains full speed regardless of whether the switch which initiates closure of those contacts is opened.

Another object is to provide improved time delay means comprising a capacitor for maintaining a relay coil energized after the switch which initiates energization of the relay coil is opened.

Another object is to provide improved control system of the aforesaid character which are particularly well adapted for use on self-propelled lift trucks, which are economical to fabricate and which are reliable and foolproof in use,

Other objects and advantages of the invention will hereinafter appear.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing illustrates a preferred embodiment of the invention but it is to be understood that the embodiment illustrated is susceptible to modification with respect to details thereof without departing from the scope of the appended claims.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawing, there is shown schematically a control system according to the invention for a hydraulic device, such as a lift ram 10 which is connected to the chassis 12 of a self-propelled lift truck. Lift ram 10 comprises a cylinder 14 and a movable piston 16 having a piston rod 18 which carries a pulley 20. A lift chain 22 which is reeved about pulley 20 has one end connected to cylinder 14 and its other end connected to a vertically movable carriage 24.

Hydraulic fluid from a source such as a hydraulic reservoir 26 can be supplied to the lower end of cylinder 14 of lift ram 10 by a pump 28 downstream of the reservoir through a pipe line 30, a load check valve 32, a manually operated control valve 34 downstream of the pump, and a pipeline 36. Control valve 34 which is shown in neutral or hold" position is operable upwardly to lower position or downwardly to lift" position by a vehicle driver through a control level 38. When valve 34 is in the illustrated neutral position, fluid from pipeline 30 can be routed through middle section 40 of valve 34 and through return pipeline 42 to reservoir 26 until pump 28 stops running. When valve 34 is raised, fluid from cylinder 14 of lift ram 10 can be routed through pipeline 36 through lower section 44 of valve 34 and through a pipeline 46 to reservoir 26, thereby allowing piston 16 and carriage 24 to descend under the force of gravity. When valve 35 is lowered, it permits fluid to flow or be routed from reservoir 26 by pump 28 through pipeline 30, check valve 32, through pipeline 36 to cylinder 14 of lift ram 10 to raise piston I6 and lift carriage 24.

Pump 28 is driven by an electric motor 50 which is selectively operable, as hereinafter explained, in response to the position of control level 38 and valve 34 which are connected to or associated with a switch 52. Switch 52, for example, comprises a pair of normally open stationary contacts 52a and 52b which can be connected by a bridge contact 520 which is carried by and movable with valve 34. Switch 52 is open when valve 34 is in neutral or raised position and is closed when valve 34 is lowered by lever 38, i.e., when it is desired that fluid be pumped to cylinder 14 ofram 10.

Switch 52 is connected to an electrical control circuit or motor control means 54 for motor 50. Motor control means 54 is operated from a source of electrical power, such as a storage battery 56, which is carried by the forklift truck, for example. Motor 50 is energizable from and is connected across battery 56 is series with normally open separable contactor contact 58a of a contact 58 which is part of the motor control means. Contact 58a is shunted by means, such as a capacitor 60, which reduces or inhibits arching on these contacts as they open. Contactor 58 further comprises a contactor coil 58b which is energizable from battery 56 and is connected thereacross in series with the normally open series connected relay contacts 62a and 62b of relay 62 which is part of the motor control means. Relay 62 further comprises a relay coil 62c which is energizable from battery 56 and is connected thereacross in series with a resistor 64 and normally open switch 52. Coil 62c of relay 62 is shunted by a capacitor 66. Capacitor 66 and resistor 64 serve as a time delay means which controls the length of time coil 62c remains energized after switch 52 is manually closed, as hereinafter explained. ln a specific embodiment of the invention battery 56 had a 36- volt output, resistor 64 was rated at 100 ohms, and capacitor 66 rated at 250 mi.

The control system hereinbefore described operates as follows. First, assume that all components are in the position or condition shown in the drawing. Then assume that it is desired to raise carriage 24 to some desired level above that shown. To accomplish this, control level 38 is manually moved downward (clockwise) to depress valve 34 and bring section 48 thereof into play and to simultaneously close switch 52. Closure of switch 52 causes energization of relay coil 62c and charging of capacitor 66. Energization of relay coil 62c causes contacts 62a and 62b to close and thereby cause energization of contactor coil 58b. Energization of contactor coil 58b causes contactor contact 58a to close and thereby connect motor 50 to power source 56. Energization of motor 50 causes operation of pump 28 and hydraulic fluid is delivered from reservoir 26 through pump 28, line 30, check valve 32, upper section 48 of valve 34, line 36 to cylinder 14 of lift ram 10. Consequently, piston 16 and carriage 24 are raised.

Since motor 50 is started against a load, namely, the hydraulic fluid in the pump and the weight of carriage 24 and whatever load it may carry, and since a time interval (from about 0.2 to 0.4 seconds) is required for the motor to reach full speed, the motor is subjected to high inrush currents from battery 56. It motor 50 is disconnected from battery 56 before it is up to speed and while inrush current flows by opening the contact 58a of contactor 58, the contact 58a would be subjected to serve arcing which damages the contacts and requires their frequent replacement. In conventional prior art systems such premature disconnection occurred whenever the operator intentionally or accidentally moved the control lever 36 to neutral or upward position before an appropriate time interval elapsed after closing of the switch, i.e., until the motor reached full speed.

In accordance with the present invention, the RC circuit comprising resistor 64 and capacitor 66 cause delay coil 62c to remain energized for a predetermined length of time (i.e., about 0.2 to 0.4 seconds) even though switch 52 is immediately opened after being closed. After the predetermined time interval has elapsed, coil 620 is deenergized (provided switch 52 has been opened and valve 34 returned to neutral or upward position) and relay contacts 62a and 62b open. By this time motor 50 is up to speed and deenergization of coil 58b and opening of contactor contact 58a can occur without the latter opening on an inrush current. The capacitor 60 across contact 58a absorbs some of the residual energy which would otherwise cause arcing, and damage to contact 58a is substantially reduced.

If control lever 38 and valve 34 are returned to neutral position and switch 52 is opened to deenergize motor 50 as hereinbefore explained, fluid being driven by pump 28 as motor 50 comes to rest is diverted through pipeline 42 to reservoir 26. Fluid cannot drain from cylinders 14 while valve 34 is in neutral position. If however, valve 34 is moved to upward position by lever 38 and switch 52 is opened, fluid driven by pump 28 as motor 50 comes to rest is diverted through pipeline 42 to reservoir 26 and carriage 24 and piston 16 descend as the fluid from cylinder 14 flows through pipelines 36 and 46 back to reservoir 26.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. in a control system for a hydraulic device,

a hydraulic fluid source,

a pump downstream of said fluid source,

a valve downstream of said pump,

said valve being movable to one position wherein it permits fluid flow to said device,

an electric power source,

a motor for driving said pump, motor control means comprising separable contacts for connecting and disconnecting said motor to and from said electric power source.

a first contactor having a contactor coil which controls operation of said separable contacts,

a relay including a relay coil and relay contacts for connecting and disconnecting said contactor coil to said electric power source,

means to effect connection of said relay coil to said electric power source including a switch movable to a first position in response to movement of said valve to said one position, and

time delay means operable in response to operation of said switch to maintain said separable contacts closed for a predetermined length of time after said valve is moved from said one position and said switch is moved from said first position including a capacitor in a circuit in shunt to said relay coil.

2. The control system according to claim 1 and further comprising a resistor connected in series with the shunt arrangement of said capacitor and relay coil.

3, A control system according to claim 2 including a capacitor connected across said separable contacts for are inhibition purposes.

4. A control system according to claim 3 which is carried on a lift truck, wherein said hydraulic device is a lift ram, wherein said valve and said switch are operable in response to movement of a manual control lever carried on said lift truck and wherein said electric power source is a battery. 

1. In a control system for a hydraulic device, a hydraulic fluid source, a pump downstream of said fluid source, a valve downstream of said pump, said valve being movable to one position wherein it permits fluid flow to said device, an electric power source, a motor for driving said pump, motor control means comprising separable contacts for connecting and disconnecting said motor to and from said electric power source. a first contactor having a contactor coil which controls operation of said separable contacts, a relay including a relay coil and relay contacts for connecting and disconnecting said contactor coil to said electric power source, means to effect connection of said relay coil to said electric power source including a switch movable to a first position in response to movement of said valve to said one position, and time delay means operable in response to operation of said switch to maintain said separable contacts closed for a predetermined length of time after said valve is moved from said one position and said switch is moved from said first position including a capacitor in a circuit in shunt to said relay coil.
 2. The control system according to claim 1 and further comprising a resistor connected in series with the shunt arrangement of said capacitor and relay coil.
 3. A control system according to claim 2 including a capacitor connected across said separable contacts for arc inhibition purposes.
 4. A control system according to claim 3 which is carried on a lift truck, wherein said hydraulic device is a lift ram, wherein said valve and said switch are operable in response to movement of a manual control lever carried on said lift truck and wherein said electric power source is a battery. 